In recent years, environmental protection guidelines in the United States and Europe have encouraged the development of alternative energy sources for engines using hydrogen as fuel. The most modern process, having the highest efficiency for in-situ production of hydrogen, is reforming alcohols or hydrocarbons, in particular methane and diesel fuel, in combination with a water gas shift reaction (WGSR) and the selective oxidation of CO (SelOx), followed by energy generation in fuel cells. Fuel cells are significantly more energy efficient than internal combustion engines. Thus, power stations using fuel cells are able to achieve a system efficiency of 70-80%, compared to 30-37% for combustion. In the transport sector, polymer membrane fuel cells (PEMFCs) or high-temperature fuel cells (SOFCs) achieve an efficiency of 40-50%, compared to internal combustion engines (IC engines) 10 which have a present-day efficiency of 20-35%.
Polymer membrane fuel cells are compact, have a high power density and can be operated at low temperatures. However, they suffer from electrode poisoning (anodic catalyst Pt, Pt-Ru) by carbon monoxide, if the concentration of carbon monoxide is above 20 ppm. It is difficult to completely eliminate CO after reforming and the water gas shift reaction; thus, a need exists for CO removal from hydrogen-containing mixtures.
The method with the best prospects is the oxidation of CO by addition of small amounts of oxygen, but this requires highly selective catalysts that can oxidize CO without the simultaneous oxidation of hydrogen at the lowest temperature possible.
Many catalysts for the selective oxidation of CO (known as preferential CO oxidation in an excess of hydrogen, “PROX”) are known. These include systems based on gold and silver catalysts. However, these systems have the disadvantage that they have both low thermal stability and low stability under reaction conditions, which results in partial deactivation of the catalyst. A further disadvantage of these systems is that they are quite sensitive to moisture and CO2.
Further known catalysts are those based on copper; these systems have only a low activity with respect to the oxidation of CO below 200° C. in the presence of hydrogen. These systems, too, are very sensitive to water and in particular, CO2.
W. S. Epling, P. K. Cheekatamarla and A. M. Lane, Chemical Engineering Journal 93 (2003) 61-68, disclose a platinum- and cobalt-based catalyst on a support structure composed of TiO2, which is used for the selective oxidation of CO (PROX). A high catalyst activity was found here, but it was not possible to achieve the complete removal of carbon monoxide under reaction conditions for polymer membrane fuel cells, which can be attributed to the low selectivity of CO.